A thermoplastic elastomer composition comprising a matrix (i.e., a continuous phase) of a thermoplastic resin, in which a rubber is finely dispersed (i.e., a dispersed phase) is known. It is known in the art that an island-in-sea structure, in which the rubber is surrounded with the resin, is formed under the conditions satisfying the formula (IV) explained later (e.g., see Japanese Patent Publication (A) No. 2000-159936). To obtain a rubbery elastomer from a composition obtained in a state where a thermoplastic resin forms a matrix, it is necessary to increase the amount of rubber as much as possible. However, when a large amount of rubber is compounded, the thermoplastic resin serving as the continuous phase and the elastomer component serving as the dispersed phase invert in phase, and, therefore, the thermoplastic elastomer composition thus prepared does not exhibit the fluidity of the thermoplastic resin and the shaping becomes impossible. Therefore, there have been limits to the amount of rubber which could be compounded.